Hot melt adhesives are thermoplastics based on polymer compositions that liquefy between temperatures of 80° C. to 220° C. and solidify again when cooled. Hot melt adhesives are desirable for their quick setting and/or the absence of aqueous or solvent media that provide fluidity to other types of adhesives. They consist of dry polymer (less than 5% liquid) and are applied in a molten state without using water or solvents. Hot melt adhesives are considered environmentally friendly and can provide a manufacturing benefit by reducing the number of steps in an operation or by allowing the conversion of an operation to automation.
Hot melt adhesives are widely used in industry for various applications. Typical applications include product assembly, packaging such as case, carton or tray forming, and in glue sticks. Desirable properties in a hot melt adhesive include suitability for high temperature end use, low temperature flexibility, low viscosity, and thermal stability.
Hot melt adhesives can be categorized according to their working temperature and their adhesion function. For a given application, often the properties, the handling and equally importantly the cost are factors in choosing a hot melt adhesive. Hot melt adhesives in use today are generally based on low molecular weight polyethylene homopolymers (LMPE), ethylene vinyl acetate copolymers (EVA), polyamides and moisture cross-linkable polyurethanes. However, polyurethane hot melt adhesives are quite expensive, and polyamide hot melt adhesives are sensitive to water and steam and require higher temperatures for handling. LMPE-based hot melt formulations can have good elevated temperature end use characteristics, but are undesirably brittle at low temperatures. EVA is one of the most versatile hot melt adhesives, with lower cost, easy handling, and good adhesion to many substrates. However, EVA copolymers generally have poor elevated temperature performance (poor temperature resistance) and are prone to char, skin or gel when exposed to elevated temperatures in the presence of air. In addition, the adhesion of EVA-based adhesives may not be adequate for more demanding applications.
U.S. Pat. No. 6,946,528 discloses a hot melt adhesive composition comprising a tubular reactor copolymer of ethylene and at least 5 mol % of comonomer units derived from alkyl acrylates, alkyl methacrylates, or mixtures thereof, wherein the copolymer has: (a) a melt index from 300-10,000 g/10 min; (b) a maximum peak melting temperature of at least 100° C.; and (c) a temperature required to melt 50% of the copolymer of at least 80° C.
U.S. patent application Publication US2006/0025527 discloses adhesive compositions comprising nonfunctionalized base resins and functionalized ethylene copolymers such as ethylene/maleic anhydride (E/MAH) or ethylene/ethyl hydrogen maleate copolymers that can be applied as self-supporting films or can be co-extruded or extrusion coated onto a substrate.
There is a need for improved hot melt adhesives that can be applied at temperatures less than 180° C. and provide good bond strength (adhesion) to a broad spectrum of substrates. The adhesives desirably provide good toughness, good elevated and low temperature performance, including good heat resistance, thermal stability, and easy flow viscosity for handling.